1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery comprising the same. More particularly, the present invention relates to a negative active material for a rechargeable lithium battery exhibiting good initial efficiency and reversible efficiency, a method of preparing the same, and a rechargeable lithium battery comprising the same.
2. Description of the Related Art
Currently, commercially available rechargeable lithium batteries exhibit higher power and energy density than the conventional nickel-cadmium battery or nickel hydrogen battery, and substantially generate 4V, i.e., they have an average discharge potential of 3.7V. Such a battery is considered an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as a mobile telephone, a notebook computer, a camcorder and so on, which are abbreviated as 3C devices.
Attempts have been made to use lithium metal, as a negative active material, for the rechargeable lithium battery because of its high energy density. However, lithium metal causes a serious problem of dendrite formation on the surface of the lithium metal during charging and discharging. This may cause a short circuit and increase the reactivity of the lithium metal so that the lithium metal reacts with an electrolyte, to form a polymer film without ionic conductivity on the surface of the lithium metal. As a result, the battery resistance increases abruptly, preventing smooth charging and discharging.
Such a problem may be addressed by replacing lithium metal with carbonaceous materials that are capable of intercalating and detintercalating lithium ions. Carbonaceous materials have no shortcomings associated with dendrites. However, the carbonaceous materials, especially graphite, have smaller theoretical capacities, such as 372 mAh/g, which is only 10% of the theoretical capacity of lithium metal.
Recently, various attempts have been made to increase theoretical capacities. One such attempt includes the use of metals or non-metals that form a compound together with lithium. For example, tin (Sn) intercalates lithium to form a Li22Sn5 compound from which lithium is not extracted, thereby causing no formation of dendrites. Thus, when a tin-included compound is electrochemical reversibly applied to a negative active material, it is expected that a theoretical capacity of 993 mAh/g may be obtained, which is significantly higher than that of the theoretical capacity of graphite.
However, metal compounds, such as the above-mentioned tin-included compound have several disadvantages. These disadvantages include poor electrochemical reversibility, poor charge and discharge efficiencies, and rapid capacity fading during charging and discharging. One of the possible causes of these disadvantages includes low electrical conductivity of the metal compounds, which in turn is caused by the formation of an oxidation layer thereon during the preparation of these metal compounds. Electrical conductivity also depends on the type of metal used.
In addition, these disadvantages may be caused by the formation of a Li-metal intermetallic compound during electrochemical charging and discharging that causes extreme expansion and contraction of the crystalline matrix volume, which in turn leads to cracks and to minute particle generation of the metal compound, thereby facilitating formation of a solid electrolyte interface layer. Such miniscule material forms spaces in the metal compound, causing a reduction in reversible discharge efficiency and discharge capacity. Therefore, there is a need to provide a negative active material for a rechargeable lithium battery that exhibits high capacity, improved discharge reversible efficiency, and capacity retention.